1. Field of the Invention
The present invention relates to a separator for an alkaline storage battery and to an alkaline storage battery using the separator.
2. Description of the Related Art
In recent years, alkaline storage batteries have been used as a power source of various devices, such as a cellular phone, a notebook computer, an electric vehicle, a hybrid vehicle, etc. An increase in characteristics, especially an increase in lifetime, of such alkaline storage batteries has been highly demanded. An alkaline storage battery includes a positive electrode plate, a negative electrode plate, a separator, and alkaline electrolyte retained by the positive electrode plate, the negative electrode plate, and the separator. In order to increase the lifetime of the battery, it is necessary to increase the liquid retaining ability of the separator such that a larger amount of the alkaline electrolyte is retained by the separator. In order to increase the liquid retaining ability of the separator, it is necessary to increase the surface area of the separator.
A typical separator used in an alkaline storage battery is porous, i.e., it has a large number of pores. The liquid retaining ability of such a separator varies according to the diameter of the pores. Electrolyte does not readily pass through a pore having a small diameter, and hence is steadily retained in such a small diameter pore; whereas electrolyte readily passes through a pore having a large diameter, and hence is not steadily retained in such a large diameter pore. Thus, since a separator of a storage battery has various pore sizes, the distribution of electrolyte inside the battery is non-uniform.
In a separator of an alkaline storage battery, for the purpose of increasing the ability for retaining electrolyte, the density (weight per unit area) of the separator is increased, or the separator is made of a finer fiber such that each pore of the separator has a small diameter. However, in the case where the diameter of a pore is reduced, the air permeability of the separator is decreased, and accordingly, the internal pressure of the battery is increased. For example, although in normal operation, gas which is generated from a positive electrode plate when the battery is overcharged is transmitted through the separator and consumed by a negative electrode plate, this reaction is suppressed when the air permeability of the separator is low, and accordingly, the internal pressure of the battery is increased. If the internal pressure of the battery exceeds the threshold pressure of a safety valve, gas inside the battery, or the like, is expelled from the battery. As a result, the lifetime of the battery may be shortened.